The present invention relates to a handling device for an assembly, particularly for a fertile or fissile assembly of a fast neutron nuclear reactor with cooling by a liquid metal.
It is known that in such a reactor the assemblies constituting the reactor core are formed by open enclosures containing a bundle of sheathed needles of fertile or fissile material. These assemblies are juxtaposed and rest in a vertical position on a supporting member. The complete core assembly is submerged in a suitable volume of liquid metal, generally sodium, contained in a container which is open at its upper end and is suspended beneath a thick slab, particularly made from concrete which extends horizontally and encloses a protective caisson surrounding the container. The liquid sodium circulates from bottom to top in the enclosures of the assemblies in contact with the needles in such a way as to extract the calories given off by the latter, the heated sodium being taken up by heat exchangers and then on leaving the latter by pumps which return the cooled sodium beneath the supporting member for a further passage through the core. In operation, the mass of liquid sodium in the container generally reaches a given level, separated from the lower face of the slab by an inert gas, generally argon cushion.
At given intervals, variable according to the reactor operating conditions, it is necessary to replace the assemblies in the core. For this purpose, the installation has transfer means controlled from the outside of the protective caisson and which after traversing the slab are able to take up and extract one by one the assemblies in the core, whereby said assemblies are laterally displaced whilst maintaining them below the sodium level and finally they are deposited in a handling pot located at a loading and unloading station located on the side of the core and which is also beneath the sodium level. The handling pot is generally formed by an elongated receptacle open at its upper end via which the assembly is introduced and closed at its inner base, said pot generally being provided with means which, during its movement to an unloading station, permit the distribution and evacuation in acceptable proportions of the residual calories given off by the assembly.
In a generally adopted construction, the handling pot is placed on a carriage against which it bears whereby after tilting said carriage is able to move in an inclined ramp by means of a cable or pull chain, thereby making it possible to remove the carriage, handling pot and the assembly contained therein from the container and caisson in order to bring them in tightly sealed manner in a chamber provided externally of said caisson to a position above the slab where the carriage is tilted and then returned over another inclined ramp. It then once again traverses the slab, externally of the caisson, and issues into an appropriate storage container where the pot is again submerged beneath an appropriate volume of liquid sodium and finally arrives at a loading and unloading station. Advantageously, the attachment straps for the carriage are provided with a parachute brake system, which prevents the sudden dropping of the pot containing the assembly in the case of an accidental breakage of the pull cable. Such a device is in particular described and claimed in French patent application No. 2,192,057.
When the carriage and the pot are in a position vertical to the loading and unloading station of the reactor or storage container the attachment straps, whose lower end is connected to the carriage and whose upper end is connected to the parachute brake which always remains on the inclined ramp, are disposed obliquely and provide vertical access to the pot in order to permit the introduction or removal of an assembly.
During an unloading operation the handling pot is initially completely submerged below the sodium level. When the carriage arrives in the vicinity of the slab, the upper end of the pot emerges above this level into the inert gas cushion and then the whole pot progressively emerges so that it is totally penetrating the gaseous atmosphere in the container above the sodium level and communicates with the inside of the chamber.
Under these conditions the pot containing the assembly must be closed so as to prevent the evaporation of sodium, which may occur in the case of an accidental immobilisation of the pot in a partly or completely emerged position as a result of the rise in the sodium temperature. The drop in the sodium level may cause the circulation by convection of the sodium in contact with the assembly to stop, resulting in a local temperature rise which may even bring about a fusion of the fuel, which constitutes an inadmissible accident at the safety level.